A single-color or multi-color prepress proof suitable for proof printing, etch, is recently increasingly produced. The prepress proof is produced by imagewise exposing an image-forming material comprising a substrate and photosensitive layer formed on one surface of the substrate and then transferring an image to an image receptor such as paper under heat, generally around 100.degree. C. The image-forming material is formed of a substrate, a photosensitive layer formed on one surface of said substrate and a protection film. As the substrate and the protection film, a thin film or sheet formed from a material such as cellulose acetate, polystyrene, polyvinyl chloride or polyethylene terephthalate is used. As the image receptor, a properly selected printing sheet such as paper or a film is used.
As an apparatus for preparing the above prepress proof, generally, there has been proposed an image-transfer apparatus having a platen to which an image-forming material is to be attached and a transfer cylinder to which an image receptor is to be attached, and the platen and the transfer cylinder are brought into contact so that a photosensitive layer of the image-forming material and an image receptor are brought into contact with each other under pressure to transfer an image formed on the photosensitive layer to the image receptor in a nip portion formed by the platen and the transfer cylinder. A multi-color image transfer product or a prepress proof sheet is produced by color-scanning an original of multi-color image, imagewise exposing image-forming materials corresponding to separated colors, e.g., yellow, magenta, cyan and black to form images of these colors in the image forming materials, and consecutively transferring these formed images to one image receptor.
As a device for attaching the image-forming material to the platen, a pin bar is generally used to facilitate the registering of each color. The pin bar has a structure in which a plurality of pins are provided on a plate extending in the axial-length direction of the platen. When, however, the pin bar is directly provided on the platen, there is caused a distinct height difference between the platen surface and the pin bar, and the image-forming material which has been attached has a nonuniform slackening. Therefore, the registering accuracy is insufficient in many cases. There is further another problem in that the area on which the printing can be actually effected is decreased since the printing with sufficient registering accuracy cannot be carried out on the pin bar portion.
For producing a multi-color prepress proof, a plurality of image-forming materials and one image receptor are used. After the imagewise exposure, each image-forming material is attached to the platen in a state in which the protection film is stacked. On the platen, the protection film is manually peeled off by an operator. However, it is difficult to peel off the thin protection film, and the peeling off the thin protection film extremely impairs the workability. Further, when the protection film is peeled off, an unexposed portion, i.e., an image portion, is sometimes peeled together, and in such a case, no aesthetically fine image transfer product can be obtained.
It has been strongly desired to develop a device for attaching a sheet-like substance such as an image-forming material without causing a failure in registering and a device with which the protection film of the image-forming material can be easily peeled off. However, no device which satisfies the above desire has been developed.
It is therefore an object of the present invention to provide an image transfer apparatus having a device with which the image-forming material can be attached to the platen without causing a failure in registering. It is further another object of the present invention to provide an image transfer apparatus having a platen equipped with a device for easily peeling off the protection film of the image-forming material and a transfer cylinder.